text:
How complete is our understanding of lift?
I'm currently studying for my PPL and one of the accepted textbooks contains the following disclaimer at the end of the Principles of Flight section on lift:

It is important to note that the forgoing explanation of lift, and its reliance on Bernoulli's theorem, is very much the 'classical' theory of lift production and the one on which the exam questions are usually based. There are differences of opinion amongst scientists on the subject....[snip]

The same book previously also describes the venturi theory which NASA discredits.
Additionally, one of my previous CFI's told me that during a previous successful job interview he had been asked to explain lift and had merely responded with "Which theory would you like me to cover?"
On the contrary - we must have an excellent understand of some components because of the way we're able to design and build such stable (and unstable when we want) aircraft. Plus, I see some absolutely incredible mathematics described on this website which seek to accurately answer complex questions.
So, my question isn't how is lift generated -it's how complete is our understanding? Which bits are still in dispute, and which bits are fully accepted?
For those interested the book in question is
"AFE - The Private Pilots Licence Course - PPL4
Principles of Flight,
Aircraft General Knowledge,
Flight Performance and Planning"
ISBN: 978-1-874783-23-7
https://www.afeonline.com/shop/private-pilot-s-licence-course-ppl-4-principles-of-flight-airc.h

have you had a look at http://aviation.stackexchange.com/q/8281/1467 and http://aviation.stackexchange.com/q/466/1467 ?

@Federico I have seen those previously, but I guess I'm after a more general overview. I appreciate that as a pilot I don't need to understand all the intricacies but I'm curious as to if someone does!

We have a very good understanding of lift

@Ethan. Really? That puts you into a small set of people who think they know. We have theories than can be modelled and tested and which fit all known measured results.  But how lift is generated is an incomplete set of theories and the best brains in the business still argue about it. Read some of the linked answers, especially on Physics SE to understand this. We can design, model and test wings and we know that they will perform in a given manner because the results match the model. Our understanding of how the results match the model is, at best, incomplete.

@Simon, it is not lack of understanding. Physicists understand fluid dynamics pretty well and lift is just a consequence of it. It is lack of simple explanation and that is simply because lift is surprisingly complex phenomenon.

As I commented on one answer in the linked question, for pilot the key thing to know that the lift is proportional to square of indicated speed and angle of attack until some point where it decreases abruptly because stall happens. The common explanations don't help that, because they don't explain stall.

Note that in engineering contexts, it's perfectly acceptable to have two models describe the same phenomena. This is not a lack of understanding, but simply two ways of looking at the same thing. For example, compare calculating the impact velocity of a dropped mass by looking at an energy balance from potential to kinetic energy, or looking at the acceleration over a certain distance. In the end, it's the same equation

Complete enough - we've never accidentally left an aircraft in the sky, and when one does come down we have a pretty good idea of why

@JanHudec I didn't say a lack of understanding.  I said incomplete understanding.  I'm very happy to be shown to be wrong but I have read no answers or articles which fully describe the physics.  There are a complex set of equations which fit the models, but the theories behind them are still open to debate.  There are many things we have matching theories for (often multiple ones) physical phenomena, e.g. electricity, gravity, light, which we can use to completely model them but we do not understand the physics behind them in a complete manner.

@JanHudec See MSalters comment below.  Even fluid dynamasists can't agree.

I highly recommend http://physics.stackexchange.com/questions/290/what-really-allows-airplanes-to-fly, which gives a very concise explanation.

I think the real answer to your question is demonstrated by how many different answers your question has generated. Clearly there are still some questions.

@ryan I believe the real question being answered here is "How complete is the understanding of lift among Aviation.SE members". :)

@BenCrowell Should I interpret this as "the scientific community has a 100% complete understanding of how aerofoils generate lift"?

@Simon The scientific community doesn't have "100% understanding" of anything, and reputable scientists and engineers have never claimed that it does. Anybody who claims they have 100% understanding is usually a crackpot, especially if they also claim  that everyone else in the scientific community is wrong.

The claim that, according to a textbook, no scientific consensus exists on the nature of lift has generated a lot of noise here.   It would be very useful to have a reference for the quoted text.   I attempted to identify the source for the quoted text with Google books, without success.

@Crosbie Yes, very fair - I have edited it in

Short answer: Yes, our understanding of lift is complete, but solving the equations for some practical cases needs more resources than what is technically sensible.
Lift is a matter of definition
First of all, lift is only one part of the aerodynamic forces. It is the component normal to the direction of airflow. Since the aircraft will distort the local flow around itself, this direction is taken ideally at an infinite distance where the air is undisturbed.
The other component is, of course, drag. It is defined as the part of the aerodynamic forces parallel to the direction of airflow.
The aerodynamic forces are the sum of all local pressures, which act orthogonally on the local surface of the airplane, and the shear forces, which act parallel to the local surface.
When aerodynamics was researched first, electric fields were new and exciting, and the same equations which help to calculate electromagnetic forces could be used to calculate aerodynamic forces. Therefore, abstract concepts like sources or sinks were used to explain aerodynamics. This made it not easier to understand, and many authors tried to find simpler explanations. Unfortunately, they were mostly too simple and not correct, but the next generation of authors would mostly copy what had been written before, so the wrong concepts were still bandied about.
To get to the bottom of it, it might help to look at lift at a molecular level:
Every air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects:

Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise.
Pressure means that air particles oscillate all the time and bounce into other air particles. The more bouncing, the more force they exert on their surroundings.
Viscosity means that air molecules, because of this oscillation, tend to assume the speed and direction of their neighbors.

All three contributions are well understood, and with the Navier-Stokes equations they can be completely mathematically expressed. What is still improving is our ability to solve these equations, and in turbulent flow the characteristic length required to capture all effects is so small that it is practically impossible to solve those equations fully with finite time and resources.
Flow over the upper side of the wing
Now to the airflow: When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Due to the acceleration, the packet will be stretched lengthwise and its pressure drops in sync with it picking up speed. Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. Once there, it will "see" that the wing below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly (because it has mass and, therefore, inertia), the packet will change course and follow the wing's contour. This requires even lower pressure, to make the molecules overcome their inertia and change direction. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction.
Note that lift can only happen if the upper contour of the wing will slope downwards and away from the initial path of the air flowing around the wing's leading edge. This could either be camber or angle of attack - both will have the same effect. Since camber allows for a gradual change of the contour, it is more efficient than angle of attack.
Flow over the lower side of the wing
A packet of air which ends up below the wing will experience less uplift and acceleration, and in the convex part of highly cambered airfoils it will experience a compression. It also has to change its flow path, because the cambered and/or inclined wing will push the air below it downwards, creating more pressure and more bouncing from above for our packet below the wing. When both packets arrive at the trailing edge, they will have picked up some downward speed.

Behind the wing, both packets will continue along their downward path for a while due to inertia and push other air below them down and sideways. Above them, this air, having been pushed sideways before, will now fill the space above our two packets. Macroscopically, this looks like two big vortices. But the air in these vortices cannot act on the wing anymore, so it will not affect drag or lift. See here for more on that effect, including pretty pictures.
Lift can be explained in several, equivalent ways
Following the picture of a pressure field outlined above, lift is the difference of pressure between upper and lower surface of the wing. The molecules will bounce against the wing skin more at the lower side than at the upper side, and the difference is lift.
Or you look at the macroscopic picture: A certain mass of air has been accelerated downwards by the wing, and this required a force to act on that air. This force is what keeps the aircraft up in the air: Lift.
If you look at the wing as a black box and only pay attention to the impulse of the inflowing and outflowing air, the wing will change the impulse by adding a downward component. The reaction force of this impulse change is lift.
Either way, you will arrive at the same result. By the way: Most of the directional change happens in the forward part of the airfoil, not at the trailing edge!

Supersonic flow
When the aircraft moves faster than pressure changes propagate through air, the changes in pressure are no longer smooth, but sudden. The aircraft will push the air molecules aside, producing a compression shock. Behind the shock front pressure, temperature and density are higher than ahead of it, and the increase is proportional to the local change in flow direction. The incremental pressure change $\delta p$ due to the aircraft hitting air with an incremental angle of $\delta\vartheta$, expressed in terms of the undisturbed flow with the index $\infty$, is proportional to the change in the streamlines:
$$\delta p = -\frac{\rho_{\infty}\cdot v^2_{\infty}}{\sqrt{Ma^2_{\infty} - 1}}\cdot\delta\vartheta$$
Gas pressure on a molecular level is the number and severity of particle collisions. The air molecules experience more collisions on the downstream side of the shock, since air pressure is higher there. The average  direction of the additional collisions is indeed orthogonal to the shock, because it is the boundary between blissfully unaware molecules at ambient pressure ahead of the shock and their bruised brethren downstream which have just crossed that boundary. Once a molecule has passed the shock, the collisions are coming again equally from all sides and its speed does not change any more.
If the surface curves away from the local flow direction, the air produces an expansion fan which re-sets the old pressure and density values when the air flows again in its original direction.
Pure supersonic lift is only a matter of the angle of incidence, and any local curvature of the wing will not change overall lift (but increase drag). Now the total aerodynamic force is normal to the wing, and drag will become proportional to the angle of incidence. In hypersonic flow you will get good results with the venerable impact theory first formulated by Isaac Newton.
Separated flow
This happens when the air molecules are no longer able to follow the contour of the aircraft. Instead, you get a chaotic, oscillating flow pattern which is very hard to compute exactly. This is really the only part of aerodynamics which cannot be predicted precisely, even though the effects are well understood. Separated flow will produce lift, too, but less than attached flow. In delta wings, this separation is produced on purpose to create what is called vortex lift.

I read somewhere that most of the lift is produced not just at the front, but front of the upper surface. Is that correct? If so, it doesn't seem consistent with your explanation of lift at the molecular level (that it's due to more molecules bouncing on the lower surface than the upper). That would suggest that the lower surface is what produces most of the lift, since that's where the net-positive bouncing happens.

@yshavit: Yes, suction is just less pressure on one side. Now it depends what you see as the direct source of lift. You can 1) either vote for suction, or 2) for impulse exchange, or 3) for pressure. All three views are equally defensible - it depends on your point of view.

Hm, okay. The reason I asked is that at the molecular level, "suction" is not a force. The only way an air molecule could suck the wing molecule up is if you had an attractive electrostatic force between them. So if the mental model is pressure, then it's really a case of the wing being pushed up from the bottom, not pulled up from the top. But it sounds like you're saying that's a valid way to think of it?

@yshavit: Yes. The wing pulls the air above itself down as much as it pushes the air below itself down, too. Now, on the molecular level, you are right to say that the air above the wing is only pushed down by pressure from above because the wing creates a barrier to pressure from below. But that is suction, only in other words.

Thanks! That's the one bit that's always confused me -- the notion that lift is generated more on the top of the wing than on the bottom. I think it makes sense now.

The answer does not cover the Kutta–Joukowski theorem, would that be an interesting addition to it?

@Orbit: I did not want to use theories but make the explanation descriptive and graphic. Adding that the flow does not go around the trailing edge because of some arcane theory is not in that spirit, I thought. I have to add that Kutta is instrumental if you need to find the right vorticity for a potential flow code but again, explaining lift with sources, sinks, doublets and vortices is a poor way to make it graphic and easily graspable, however well it works mathematically.

@PeterKämpf I wasn't aiming at the trailing, more the general idea that lift is related to rotation. I thought maybe there are still some things there that are not completely understood(maybe because I found it difficult to understand). I looked in to it a bit again, and I was mistaken, lift is not really caused by rotation, rotation is just a way to calculate it more easily.

@Orbit: Rotation is only part of it. Superimpose it on translation and you get the flow bending I am talking about. But it is harder to grasp that way, even if the superimposition is linear. Kutta gives you the right amount of rotation to superimpose on that translation.

@PeterKämpf I think it is easier to understand when superimposed, at the lower side the flow is slowed down and at the upper side it speeds up. It makes sense that this creates lift. Thanks for taking the time to respond, it's nice to finally understand the concept, even though it is 15 years after my exam. I think I will leave it at this and not dig in to the complex derivation of the theorem at this point.

I'm surprised there are so many answers saying we understand it very well when we are having trouble with things at smaller scales such as humminbirds and especially insects.

@DKNguyen What trouble in particular?

@PeterKämpf My understanding is that we do not model the unsteady state flow accurately involved with insects, nor really understand what is happening.

@DKNguyen: Unsteady flow simulation is computationally demanding. That not more has been done has to do with the limited budgets and access of biologists for high performance computing. What I find (link, link, however, shows good agreement between experiment and computations. We do understand what is happening, at least when you read more recent research.

Be aware that at the molecular level, ALL forces are just the force exerted by the electromagnetic interaction between the electrons in the outer shells of the molecules interacting with each other when they close to each other.  There are only four forces in the universe. Electromagnetic force (the strongest), The "strong" force, responsible for  interactions within the nucleus of an atom, the "weak" force, responsible for the stability of the quarks within protons, neutrons and other hadrons, finally, the weakest force, gravity.  Every other theory of Lift is derived from that.

At the most basic level, (the exchange of momentum when two particles approach and rebound due to their electromagnetic repulsion), we understand that completely (although what exactly causes electromagnetic force is still a mystery). Every other theory of lift (INCLUDING NAVIER STOKES), simply involves calculating aggregate values as a result of the trillions of molecular collisions that are the source of the force. When we define a new abstract "force" to be useful in a particular scenario,  or to solve a specific problem, we sometimes do not understand how to calculate it.

From this paper:

The principle of
equal transit times holds only for a wing with zero lift. [!!]

[...]

The air passes over the wing and is bent down. Newton’s
first law says that them [sic] must be a force on the air to bend it down (the action). Newton’s third
law says that there must be an equal and opposite force (up) on the wing (the reaction). To
generate lift a wing must divert lots of air down.

[...]

So how does a thin wing divert so much air? When the air is bent around the top of the wing, it
pulls on the air above it accelerating that air downward. Otherwise there would be voids in the
air above the wing. Air is pulled from above. This pulling causes the pressure to become lower
above the wing. It is the acceleration of the air above the wing in the downward direction that
gives lift.

We (those of us reading this) can conlude the following about our (humanity's in general) understanding of lift:

We certainly understand it well enough to design aircraft, and there may be overlap with this knowledge in other areas, such as maybe wind-powered generator design.
Many believe we have a fairly complete understanding of lift.

The second bullet is not at all in impune the excellent (and challenging!) work done through history in fluid dynamics, aeronautical physics, and aeronautical engineering. It is merely to allow for the possibility of future paradigm shifts in our understanding of those topics, even if those shifts do not affect common design practice or practical discussions of lift. A historical example of that last point would be General Relativity as a paradigm shift in our understanding of gravity, while Newtonian gravitational theory was still used for the moon program and is still widely taught and used for situations not requiring extreme precision.
In addition to links in Frederico's comment, see also: https://physics.stackexchange.com/questions/290/what-really-allows-airplanes-to-fly
Edit:
This NASA page discusses the controversy of "Bernoulli versus Newton" and concludes that both explanations of lift are "correct" and that there is even more to it. The Euler Equations and the Navier-Stokes Equations are mentioned. This page in the same series on NASA's site suggest that lift is fairly well understood by experts, but is poorly explained in the majority of popular sources:

There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments. To help you understand lift and its origins, a series of pages will describe the various theories and how some of the popular theories fail.
Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.

The edit is quite relevant. No physicist would doubt Navier-Stokes. The problem is that Navier-Stokes is fiendishly difficult and far too general for something as simple as lift. (yes, lift is simple in comparison to Navier-Stokes). And even so, a good physicist knows deep in his heart that Navier-Stokes is still wrong because it assumes fluids are not made up from molecules. The theory breaks down at microscopic scales.

@MSalters, no, lift is not simple in comparison to Navier-Stokes. Explaining lift requires both inertia and viscosity and Navier-Stokes are just expression of those (plus the appropriate conservation laws).

@MSalters, the fact that Navier-Stokes breaks down at microscopic level does not make it wrong. It merely makes it an approximation. All thermodynamics is like that.

Missing "This paper" link on comcast, see http://web.archive.org/web/20140330015638/home.comcast.net/~clipper-108/lift.pdf Also note that Navier-Stokes usually provides no explicit solutions, and instead we rely upon NS-based numerical simulations (and adding more supercomputers, to increase resolution and reduce errors.)  "If you can't explain it to your grandmother, it means that you don't actually understand it."  Heh. So, we'd finally have full understanding, only if we could successfully explain it to little kids, and to non-tech adults.

The problem here is that "correlation does not imply causation". Neither Bernouilli's principle nor Newton's laws of motion explain lift. Both of them give valid methods of calculating the lift force from the air flow pattern around the wing, but neither of them explain why the flow pattern is what it is.
Ideas like "equal transit time" at least try to give a reason "why," but experiments which visualize the flow pattern with smoke demonstrate that is just wrong.
The best "one-word explanation" of what causes lift is the viscosity of the air. Viscosity is the reason why there can't be any discontinuities in the overall flow pattern*. In particular, the air velocities on either side of the relatively sharp trailing edge of the wing have to be the same, otherwise the effect of viscosity at that point would propagate upstream through the air (at the speed of sound) and change the global flow pattern.
If there were no viscosity, no wing of any shape would produce any lift, or any drag force.
*Let's limit this discussion to subsonic flows. Introducing shock waves into the airflow makes a "hand-waving" non-mathematical discussion more complicated, but it doesn't invalidate the essential point I'm trying to make.

Well, you also need inertia, otherwise the flow would not continue downward in the direction of the trailing edge.

Do you have a source for the claim that without viscosity, there could be no lift? Superfluid helium has a viscosity of zero, would a sheet slicing through it at an angle not experience any lift?

I would add that the Euler equations, which assume inviscid fluids, certainly do allow one to approximate the lift generated by a body in flow. This is possible because the effects of viscosity are mostly confined to the infamous boundary layer, whose effects are negligible for an aerofoil in air at modest angles of attack. It is correct that without viscosity you will find zero drag, and it is also impossible for flow to become detached from a surface ("stall" in aircraft).

@romkyns There is no net force on a body in an inviscid fluid, unless the flow contains some circulation (i.e. a vortex) around the body. But with no viscosity, there is no "simple" way to create the vortex. You can get a rough estimate of the lift on a wing by making two assumptions: (1) the air is inviscid, and (2) there is a vortex of the required strength to make the flow match up on the top and bottom surfaces at the trailing edge. Assumption 2 is the same as assuming the flow is faster over the top than the bottom. See https://en.wikipedia.org/wiki/Kutta%E2%80%93Joukowski_theorem

Even flow across shocks is continuous (i.e., shockwaves have finite thickness); they're just so thin that their thickness is often negligible when calculating larger-scale flow fields.

Not true. Lift is a force. Force is the F in F = ma, and this equation is a simplification of what Force really is. It is simply the change in Momentum when two molecules interact with one another electromagnetically.  the true equation is F = d(mV)/ dt.  And so the idea that viscosity causes lift is silly, as the only way that could happen is if there was no electromagnetic force. Which is silly, because without the EM Force,  molecules would simply ignore one another, like neutrinos.  A better answer, (clearer and more intuitive than viscosity) is that the electromagnetic force causes lift.

@AlephZero,  Viscosity and Lift are both caused by the Electromagnetic Force between atoms, Neither is the cause of the other. Lift is a force on an airfoil. Viscosity is a fore between molecules. How can they be causal? As you note early in your answer, when two things occur together, all that can be concluded is that they are correlated. Which one might be the cause of the other remains a question. In fact, they could both be the result of some unknown third factor (Maybe the EM force???)

Besides viscosity, if parcels have no mass, no inertia, then accelerating parcels produces zero force, zero Lift.  Airfoils stop working in low-R environments where inertia is irrelevant (bacteria-scale,) but work very well where parcel-inertia dominates the physics. Parcels being flung uniformly off the sharp trailing edge must continue in the same direction, because of their own inertia, and this is the source of the differing upper/lower velocities.  Diagram of an airfoil w/viscosity but no inertia http://www.av8n.com/how/img48/nocirculation.png , produces zero lift.

How complete is our understanding?

Complete enough to design and fly a number of complex aircraft of varying sizes, shapes and applications.
Complete enough to extract power using it.

On a basic level, lift is the force generated as solid body 'turns' fluid while satisfying the conservation laws. The issue is not that we don't know what lift is, but there is no consensus about how to explain it. Most of the 'theories' of lift are just models try to explain the same thing based on the points of view of the people involved.
The way the pilot views the lift is different from that of an engineer or a mathematician. For the pilot, the lift is a force that keeps the aircraft in the air (and proportional to $\rho V^{2} S$ and angle of attack, at-least till stall), while a mathematician can say that lift 'follows naturally' by solving Navier-Stokes Equation (whether it can realistically solved or not is another matter) for some conditions. Of course, this is of no practical use to either the engineer or the pilot. Both can claim (rightfully) that they are correct, while a physicist can object that NS assumes that the fluid is a continuum, while it is not the case in realty.
This is the reason for so many theories of lift. As the fluid flow is extremely complex, some simplification is done in each theory (like omitting viscosity in the Euler or potential flow theory). Based on the simplification, the theory is either useful in some (or most) of the situations or is outright wrong.

Which bits are still in dispute, and which bits are fully accepted?

Almost all 'theories' of lift accept that lift is a force and its requirements. As far as engineering goes, the issue is which bits of are necessary for the problem in hand.
For example, the potential flow theory can predict the lift as long as we are not approaching stall. After that all bets are off. There is no point in arguing about a result from a theory after using it in a situation for which it was not designed for in the first place.
This is the reason for arguments about lift. Some theories are developed to describe a particular situation (for example inviscid flow) and then applied in general, which obviously leads to confusion and dispute.
As far as engineering is considered we have enough understanding of lift to create the flying machines we need though not as much as to explain everything that happens with accuracy.

Many physical phenemona do not reduce down to an elementary algebraic equation.
Aerodynamics -- including the generation of lift -- is very well understood.  It has also been validated through experiment (many wind tunnel tests and flight tests) where the theory and experiment are in very close agreement.  In fact, we have several approaches to the development of the aerodynamic equations -- depending on the assumptions and simplifications we wish to make.
Engineers have a very good understanding of how these levels of theory compare and which are better / worse at predicting reality in different situations.
The issue at hand is that the fundamental theory of aerodynamics does not boil down to a simple equation with immediately obvious physical interpretation.
Everything in aerodynamics can be explained in terms of (and rigorously derived from):

Conservation of mass
Conservation of momentum
Conservation of energy
Gas equation of state
Some other gas relations like the dependence of viscosity on temperature

These five things are intuitive, observable, testable, etc.  There are multiple ways to get from these five things to aerodynamics....
Unfortunately, they are not simple algebraic equations.  Instead, they are a coupled system of nonlinear partial differential equations (1-3) (with some algebraic equations thrown in (4,5)).
The 'modern' way uses the least mathematical abstraction.  It is called CFD (computational fluid dynamics).  In CFD, we divide the domain of interest into a large number of tiny elements.  We initialize all of the elements to an initial guess (that is wrong).  We then solve the above equations on every element, applying appropriate boundary conditions.  This makes a small correction to the properties at every element.  We then do it again, and again, and again....
CFD is a very brute force approach with very little mathematical abstraction.  After millions and millions of calculations, if we're lucky, the properties in all of the elements stops changing (there is theory on how this should happen too).  We then say that the solution is converged.  We can then sum pressures and shear to get lift and drag, we can produce beautiful images of the flow, etc.  We have 'solved' the flow.
Although CFD is very powerful, it is not very satisfying in terms of intuition (which is what you're after).
There are other ways to attack aerodynamics (and the calculation of lift).  You mentioned Bernoulli's equation -- Bernoulli's equation is most frequently derived from the conservation of energy, but it also can be derived from the conservation of momentum.
Potential flow theory (which most pre-CFD low-speed aerodynamics theory was based on) is usually derived from the conservation of mass -- but can also be derived from the conservation of momentum.
Potential flow theory allows aerodynamic calculation with much less brute force than CFD -- but instead, it adopts a great deal of mathematical abstraction.  Complex variables (imaginary numbers), a potential function, sources, sinks, and vortices (mathematical singularities), superposition of elementary solutions, etc.
All of this can be rigorously derived from the fundamental equations -- and all of it has been validated through comparison to experiment.  And more than 100 years of successful application to the design of aircraft.  Engineers have deep understanding of their applicability and limits.
Unfortunately, although all of this is true, it does not mean that it can be explained in one or two sentences without heaps of math.
Any simple explanation of lift is wrong.
Any rigorous and correct explanation of lift is going to get messy.

Thanks for adding this excellent answer to an active question, so it won't be lost in the mass of closed, low scoring questions.  It's a great answer that adds clarity to this discussion (as well as to the other, now closed-as-dupe).

If we focus on physics misconceptions commonly believed by aerodynamics people, we might be able to remove them from lifting-force discussions.  That would let us craft some simple, intuitive explanations.  Yes, the math remains messy. For example, explain a hovering helicopter with simple prose  ...then try to calculate the lift via Navier-Stokes, and Prandtl's screw-shaped horseshoe, with full-blown vorticity-creation.  Not so easy, compared to "the helicopter's cylinder-shaped downwash is a momentum-carrying massive jet, and the helo rotor is a reaction-engine employing fluid propulsion."

TL;DR: we can very precisely model aerodynamic forces at the micro level; we can reasonably predict behavior at the macro level by aggregating micro-level models (CFD). We don't have a universally-applicable story for why the macro level behavior is what it is.
Fuller explanation:
At the risk of being a bit pedantic, I'm going to back up a couple of steps of abstraction in order to provide a more-complete picture.
The overall aerodynamic force on a body is decomposed into vectors normal to the direction of travel and parallel to the direction of travel, which are labeled 'lift' and 'drag' respectively; they are not distinct forces in & of themselves. Aerodynamic force itself is often decomposed at a different scale into pressure & friction; for the most part, friction only contributes to the 'drag' component while pressure contributes to both the 'lift' & 'drag' components.
Trying to tell a stylized story about why the integrated pressure & friction across the entire body result in a particular net force is challenging at best, since it is affected by the idiosyncrasies of each body; various models (such as Venturi, downwash, & circulation) really just provide designers & analysts with rough rules of thumb within particular flight regimes.
This last point is more important than it appears. As soon as you enter transonic flight (a mix of subsonic & supersonic flow at the surface of the body), drag increases precipitously (standing shocks creating adverse pressure gradients). Passing through to fully-supersonic flight you find yet another set of behaviors (because the leading shock radically alters the pressure distribution on the body). Don't even get me started on hypersonic flow (where the temperature change across the shocks is enough to decompose the N2 & O2 from the air itself).
Edit Peter Kampf's answer covered most of the same topics as mine, with pictures, so I'll just add this for completeness:

Scientifically speaking, lift is perfectly understood. Lift is merely the vertical component of force generated by a body moving through a fluid. And we know perfectly well how to calculate forces on a body moving through a fluid since the Navier-Stokes equations was published in 1822. That is to say, we know the physics of it and it has to do with the viscosity of fluids (in the case of aircraft, air).
But using the Navier-Stokes equations to design a wing is like trying to use Quantum-Electrodynamics (QED) to cook the perfect steak. Since gravity isn't involved in the perfectionness of steak, all you need to formulate a perfect steak is QED.
The Navier-Stokes equations calculate forces on a single point on the wing. Therefore you have to repeat the calculations over the whole wing to calculate lift. Over the last 190+ years mathematicians and engineers have formulated simpler algorithms to calculate the result of the Navier-Stokes equations and over the last 30 or so years we've used computers to calculate lift. However, you can see how this doesn't tell you the ideal shape to generate the aerodynamic characteristics you want. You can also see how this doesn't explain "lift" in terms a human can understand. It's all just large arrays of numbers.
Is it possible to explain lift in terms a human can understand? Maybe. We've certainly given names to how certain shapes generate certain output when subjected to the Navier-Stokes equations. Names such as "Coanda effect" and "Bernoulli Principle" etc. In the end, nature/physics doesn't care what name we give to our interpretation of the result of the Navier-Stokes equations - if calculating the equations result in a vertical force vector upwards you have lift. Maybe, like quantum physics, we'll never get a complete intuitive understanding of what lift is. But we certainly have the complete theory to explain it.

Additional note: Apart from not being helpful in helping us formulate a theory of wing design, the Navier-Stokes equations are also problematic because it's computationally expensive. For example, it's often not practical to use the Navier-Stokes equations to simulate turbulence (even though it's possible in theory). So we often take shortcuts for certain forms of simulations using other simpler but less perfect equations.

+1 for adressing the difference between "having a complete theory" on one side and "having an intuitive understanding" on the other.

I'm surprised there are so many answers saying we understand it very well when we are having trouble with things at smaller scales such as hummingbirds and especially insects.

@DKNguyen The Navier-Stokes equations work perfectly well to simulate the flight of hummingbirds and insects. It even works well to describe smaller things like the movement of blood in arteries. However until recently (around the 1990s) we didn't have computers fast enough to do the calculations that simulate hummingbird flight (especially since the entire system depends a lot on turbulence and vortices)

@DKNguyen actually, just in the 1990s was insect flight finally understood, when a major mystery was solved by C. Ellington's team at Cambridge, using a "Robo-moth," large flapping wings to duplicate Hawkmoth flight. They discovered a new unsuspected leading-edge vortex, produced by interaction between insect body and multiple wing-flaps, which never showed up in earlier work.  Calculations gave insufficient lift to explain moth flight. Wind tunnels, rather than NS equations, solved the mystery. Only NOW can we do the proper calcs.  https://www.google.com/search?q=ellington+hawkmoth+robo

Lift is generated because air molecules are bouncing into and rebounding from the airfoil, on both top and bottom surfaces.  It is the difference in the amount of momentum transferred in these collisions that creates lift. It is, (obviously), only the velocity of the air molecules that is normal (perpendicular) to the airfoil, that produces lift.
The Bernoulli principal is true, because the TOTAL average momentum of any air molecule in incompressible (subsonic) flow is a constant. Therefore, if the velocity of the air parallel with the airfoil increases, the normal component of the velocity must decrease to keep the total constant.
So, if the air is moving faster, the normal component must be slower, and it's pressure (against the airfoil) must be lower.
So, the longer distance to travel argument is only bogus if you try to assume that it can only be generated by a asymmetrical airfoil.  Other things can change the travel distance (and resultant  velocity) of the air across the airfoil as well. If a symmetrical airfoil is inclined to the relative wind, then as the air flows across the airfoil on the side where the airfoil bends away from the flow, the air must travel a longer distance (to fill in the gap created by the inclination) than air flowing across the surface on the other side, where the surface is inclined into the relative wind, and must either compress (in supersonic flow) or move away from (change direction away) the airfoil (in subsonic flow).
This is because in subsonic, (incompressible), flow, the air cannot make an instantaneous change in direction when it gets to the leading edge of the airfoil. If the Angle of Attack (AOA) was 10 degrees, the air does not make an instantaneous 10 degree change in direction. From the point of the leading edge away from the airfoil, the change in direction, and the resultant pressure, gradually changes as you move further away. the result is that the flow of the air is following a curved path, and travelling a longer distance, on this side of the airfoil, than it is on the other side, even for a symmetrical airfoil.

"Path length" is irrelevant, because split parcels never re-join. Instead, the top/bottom parcels remain permanently split, and the upper parcel vastly overshoots: a wider split for higher lift.  (And, if split parcels did rejoin, then the Circulation is zero, and the total lift is also zero.)  See the physics behind this at JS Denker's "See How It Flies," http://www.av8n.com/how/htm/airfoils.html#sec-other-fallacies , also see https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/learn-about-aerodynamics/#lift which uses flow-turning determined by trailing-edge flow, not by path-length.

Another note:  Bernoulli does apply to venturies such as ground-effect flight and to wings of infinite span.  But when energy is injected into parcels, as with helicopters and finite-span wings (and with birds, fish, insects etc.,) then Bernoulli/Euler Eqns. fail and cannot be employed.  Parcels with varying energy are an enormous problem.  Put simply, real wings fly by energy-injection and vortex-shedding, where only Navier-Stokes is still accurate.  Or, just explain lift for a hovering helicopter.  Then fixwing aircraft are explained by letting your hovering helicopter translate sideways.

The principles of aerodynamics and fluid dynamics are what you would call "well understood."
The ambiguity is around what so-called "lift" is, which can be a nebulous concept. For example, if you drop a piece of paper it will drift slowly to the ground, essentially a form of gliding; this same air resistance is the basic force keeping a plane aloft. Would you consider this "lift"? Once you get into these arguments about semantics, things get vague.
To prove my point, in 2020 Scientific American actually published an article entitled "No One Can Explain Why Planes Stay in the Air".
Just as one example of the craziness, the FAA test, the same one you are taking, requires you to know the "four forces of flight" in which the so-called "lift" is the force that keeps the aircraft aloft. The only problem is that you can compute lift by equations that are in every book on aerodynamics and if you actually do this (like I did) you will find that the force generated is nowhere near enough to keep a plane in the sky. If "lift" were the force keeping a plane up, it would fall like a rock, so the FAA guidelines are simply completely wrong. It's just a huge semantic hairball that is not going to go away anytime soon.
The worst part of it is that EVERY pilot (or wannabe pilot) I have ever known thinks they know exactly what "lift" is and, even worse, their beliefs generally fall into one of 5 or 6 different categories with contradictory principles. This leads to huge arguments whenever the subject comes up. After 15 years of this, I just try to stay out of it, other than to tell the beginners not to make the same mistake (like I am telling you now).

We use definitions to make clear what "lift" and "drag" are. Is's actually quite simple: Lift is the force orthogonal to the direction of flow, and drag is the force parallel to it. What brakes the fall of the piece of paper is drag, not lift.

Yeah, yeah, I have heard it all before. I have had these conversations with aeronautical engineers from MIT. The whole lift concept is completely screwed up. I am sure you have your opinion what lift is, but the TEXTBOOK the OP mentioned says there are differences of opinion, a book written by professional experts, so don't start preaching like you know the truth. The reality is that it is an ambiguously defined term.

One reason that it might seem a "screwed up" or "vague" concept is because the notion of describing "lift" as a single number is more or less useless for designing aircraft, though it is quite useful for the very simplistic explanations that are all you need to be able to fly them safely. All a pilot really needs to know about lift is, "if the plane does this, then move the controls like that to correct the situation". Similarly, a car driver doesn't need to know anything about tire design and friction to correct a skid by steering into it and not braking heavily.

Nobody seems to ever ask the questions "WHY is lift so screwed up?"  or  "WHAT CAUSES all this controversy?"   And also, why did Scientific American publish a recent article saying that lift is all screwed up (then all sorts of experts angrily responded, saying it's not?)  For deep insights, simply stop explaining, and instead answer these questions.  One answer:  look to widespread endemic misconceptions, some from 1924, and even taught by textbooks, and believed by some aerodynamics experts.  One book by physicists, "Understanding Flight" attacks common misconceptions re. lifting-force.

You succeeded in worsening your answers quoting one of the most useless article of "Scientific" American...

@sophit Here is another "worthless" article on the ambiguity of the term by Flying magazine: https://www.flyingmag.com/technicalities-losing-lift/. I guess all these people need you to educate them and tell them how to write their worthless magazines.

When a body interacts with a fluid, actually only one single aerodynamic force is generated, full stop. To simplify things when doing arithmetic, in aerodynamics that force is decomposed into a component perpendicular to the freestream termed lift and a components aligned with the freestream termed drag. As simple as that. The semantics point of view of your article is technically irrelevant. The Scientific American's article is worthless for the simple reason that it fails in understanding the basic thing that Bernoulli and Newton are exactly the same thing and not two competing explanations


text:
Who to call for flight following?
I was up this past weekend on my first solo cross country and decided to request VFR flight following and was a bit confused at what happened. For this question I will set up the actual situation as an example.
I was over the Pottstown VOR (PTW) heading to Lancaster (KLNS) on the 269 radial. I decided to call Reading (KRDG) approach for flight following as they were the closest approach or so I thought. They informed me that I was still in the Philadelphia airspace and advised to contact them. Now in the past (with my instructor) on the same flight in just about the same location we had taken flight following from Reading approach.
So,
How do you know who to hail for flight following?
I was in the Philadelphia Mode C veil, is that why they sent me to Philly Approach?

The responsibilities change over time and with weather. There's nothing wrong with starting what is closest to you, and then following the directions. It's not that you did anything "wrong" by contacting a facility that switches you to another. "approach center" is not the correct term, Center is something else.

Being inside a Class B airport's Mode C veil is a good indication you're in their approach/departure control's airspace. There's some overlap in both coverage and responsibility though, so it's worth taking a look at the other available information before making the call, if only to avoid the "You're not in my airspace, go talk to someone else" reply from the controller.

When you request flight following generally you contact the facility (either approach or center) responsible for the airspace you're in, so your real question is "How the heck do I know whose airspace I'm in?!"
The answer can usually be found on the charts, or in other FAA publications.
Let's look at your example (I was over the Pottstown VOR (PTW) heading to Lancaster (KLNS) on the 269 radial.) in detail with some convenient visual aids.

Preflight Planning and the A/FD
You don't tell us what airport you took off from, but I'm going to assume KPTW (Heritage Airport) as a convenient starting point.
If we have a look in the Airport/Facility Directory for KPTW we'll get an answer very quickly from the "Communications" section:

PHILADELPHIA APP/DEP CON 126.85 (800-354-9884)

So if we're planning a flight out of KPTW and intend to pick up flight following our call to ATC should be made to Philadelphia Approach, on 126.85 (and if we're going IFR and need to talk to them on the ground there's a convenient phone number).
If you have an application like Foreflight or Garmin Pilot this information is readily available to you in the cockpit as well, so if you're unsure who to contact you can always pull up the A/FD information for the nearest airport and use that as a starting point.

In-Flight - Using the Charts
Let's say you didn't remember to look in the A/FD to see who you needed to talk to while you were on the ground, and the battery in your iPad is dead so Foreflight is out of the picture in the air. You can pull out your paper charts and get most of the same information in flight.
If we plot PTW ... KLNS it puts you on the very bottom of the New York Sectional Chart, and on the Philadelphia Terminal Area Chart for part of the flight, so let's take a closer look at the route on each of those.
First the New York Sectional:

Not much help here - or at least nothing glaringly obvious.
Let's look at the Philadelphia TAC:

AHA! There are helpful white boxes with blue text on this chart that tell you CTC PHILADELPHIA APP ON and list frequencies, altitudes, and active runways.
This looks promising: Since the little box says to contact Philadelphia Approach that's who you should probably call first.
In your case there happen to be two helpful boxes, one above your route of flight and one below it. Both boxes are roughly equidistant from the route of flight, so it's not obvious which one you should pay attention to.   In cases like this you can call on either frequency (at least you're calling the right facility - they may make you call on a different frequency and talk to another controller, but you're in the right neighborhood at least.

You can also go to the Airport/Facility Directory (A/FD) as a tie-breaker (though you're probably less likely to pull the book out mid-air, and if you did you'd probably use the nearest airport like I described above).
The blue boxes are talking about the Philadelphia Class B airspace, so if we look at the entry for the Class B airport (Philadelphia International - KPHL) we'll get more details on which radio frequencies we should use:

If you're flying along the 269 radial from PTW you're somewhere "northwest of the airport", so you would call Philadelphia Approach on 124.35 / 128.4 if you're above 5,000 feet, or 126.85 (the same frequency we got from the A/FD entry for KPTW)if you're below 5,000 feet.

I'd Like To Phone A Friend
There's one final way to figure out who you should talk to, and that's to call someone else and ask. If you're opening your flight plan with Flight Service over the radio they will usually sign off with "For flight following try Philadelphia Approach on 126.85" and the usual plea for PIREPS, and if they don't offer a frequency you can always ask them.

Perfect Explanation! for what its worth I took of from DYL

In addition to voretaq7's excellent answer on how to figure out whom to contact in the air, it should be pointed out that if you're taking off from a towered airport, it is better to request FF from Clearance Delivery (or Ground, if they don't have CD) prior to taxi.  They'll give you your squawk code plus the frequency (Departure, Approach or Center, depending) to use when Tower hands you off, so you can have it ready on the radio instead of fumbling with charts and knobs in the air.  Once you're in the system, ATC generally won't drop you, whereas they might decide they're too busy for a cold call in the air.

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text:
What are the potential threats of having the landing gear dirt from mud in a rectractable landing gear airplane?
Recently I have been flying an aircraft with the landing gear very dirt with mud due to the parking area of the airfield on grass and the meteo conditions in recent days. I wonder what are the threats to have the landing gear with mud on it especially with retractable landing gear system as I had that day. Just for info I was flying a PA28R-201.

The main issues would be mud interfering with the uplock/downlock mechanisms, the switches that control indication of the function of those mechanisms, the brakes, and wheel balance.   Mud + freezing temperatures = worse.  Gears won't lock down, or they lock down but you don't get an indication, or the wheels freeze and skid on landing and blow tires, or mud in the wheel rim makes the wheel vibrate all to hell on takeoff.
There is usually enough robustness in the design that it can tolerate some contamination (obviously, it has to have some ability to take on mud on takeoffs from dirt runways), so as with most things, it depends how much when you look at it.  A little bit of mud spray here and there, not too big a deal.  Mud caked on on the wheels, brake calipers, any of the mechanical bits in the wheel well, that could interfere with moving parts, it should be removed before flying by hosing it off.
The mechanic who maintains the airplane you are renting would be a good person to ask about how to judge when mud contamination is excessive.

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text:
What will happen on Airbus A320-200 series when during push back NW STRG DISC in not in the towing position?
During pushback what might happen if the NW STRG DISC is not indicated green on ECAM?
Will I have an ECAM warning?
What would be the worst case scenario that might happen?

Usually it's a matter of the risk of exceeding the steering limits with the towbar if the nosewheel is not free to fully caster.  The result is normally a broken tow bar safety shear pin. Hard to believe the condition is not made obvious to the ground crew somehow.

The name says it all. NW STRG DISC or in full form the Nose wheel steering disconnect. And that is what exactly it does. It physically disconnects the nose wheel steering of the aircraft. In most aircraft, with nose wheel steering disabled, the wheel is allowed to caster. This allows pilots to maneuver the aircraft with the differential braking in case the nose wheel steering fails during taxi. So, yes even an aircraft the size of an A320 can be steered like a small aircraft with a castering nose wheel. So, a tow truck can turn the nose wheel of an A320 just like that.
The problem is that during push back the aircraft engines are started. With the engines started the aircraft hydraulics are powered and pressure is sent for nose wheel steering. The nose wheel, without any inputs always centers it self. During the push back if the nose centers it self, it can damage both the tow truck and the nose wheel of the aircraft. To prevent this the nose wheel steering has to be disconnected for push back. In the A320, there is a lever on the nose wheel which can be put in the towing position. Once it is placed in position, a bypass pin is inserted to ensure that the lever is held in place. Once the lever is positioned an ECAM memo NW STRG DISC is displayed in the cockpit in green. The message turns to amber colour when one of the aircraft engines are started. The whole point of the ECAM message is to increase pilot awareness so that you do not mistakenly leave the lever in tow position before commencing the taxi.

An Airbus A320 with nose wheel bypass pin inserted.

@CGCampbell I have changed the picture and the current one was clicked by me.


text:
Where to find approach plates for European aerodromes?
Where do I find the approach plates for European aerodromes? Is there a website intended for pilots updated and certified from the authority?

Eurocontrol has the charts for all participating countries. You'll need to register for a free account.
https://eadbasic.ead-it.com/cms-eadbasic/opencms/en/login/ead-basic/
"The EAD Basic solution is specifically targeted at:

private pilots and general aviation;
members of the public searching for aeronautical information;
training schools;
everyone who has an interest in aviation and aeronautical information."

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text:
What is Mode B on the transponder?
Mode A selected on the transponder provides identification of the aircraft to the secondary radar; Mode C will add an automatic height read-out of an aircraft. Mode S is a selective addressing. But previously there was a Mode B which nowadays is not in used anymore. What was it used for, and what information was this mode able to provide?

All modes
From a 1984 edition of Doc 9426 (ATS):

For civil aviation purposes, ICAO has designated four, differently spaced, pairs of interrogator pulses known as Modes A, B, C and D [...]
Currently both Modes B and D are reserved for future international use with the stipulation that, if future expansion requires the use of an additional mode, Mode D will be used ahead of Mode B.

SSR modes B and D haven't been used, except mode B in UK, Australia and some other countries for some time. From Radar Theory for Area/Approach Radar Controllers:

In reply to interrogation in Mode-A and Mode-B information of identity, which is set by the pilot, is sent to the ground interrogator.

ICAO Doc 9684 which is the reference for SSR doesn't even mention modes B and D anymore.
Mode B specificity
Mode B used a side-lobe suppression (SLS) feature in order to prevent transponders to reply unless they were receiving the interrogation via the main lobe signal (side lobe signals are undesired by-products of the radiation pattern). This was an improvement over the initial mode A which standard was published 3 years ago, and revised a year ago:

Source: FAA
To perform SLS, an additional pulse (P2) was sent by an omnidirectional antenna on the ground, its power was a bit more than the power of the side-lobes, but less than the power in the main lobe.

Source: FAA
The transponder did reply only when the interrogation pulses were stronger than this additional pulse. The difference on the display (left: with SLS, right: without SLS):

Source: FAA
It was soon replaced by mode C to include altitude telemetry. Mode C had SLS included. For a transponder handling multiple modes, the correct mode must be selected in parallel of the squawk code.
The interrogator (SSR ground station) can request identifiers using one mode at a time, and receive answers from transponders set to that mode. Aircraft using code 1234 will only reply when interrogated using the mode their are currently set to.
Military and civil modes identification pulses
SSR interrogation modes are identified by the timing between pulse P1 and P3 (P2 is the SLS pulse).

SSR modes identification, adapted from source
While mode A is compliant with military mode 3 (both often referred to as 3/A), mode B is purely civil.
SSR has transitioned to mode S for a large part of the centers. Mode S inherits some of the features of the legacy modes, but focuses on data exchange between interrogator and transponder. Pulses play a secondary role, data are exchanged using differentially-coded BPSK. In this system, there is little benefit of using "modes", the content of the dataframe being self-identified.

Additional information about mode B experiments:

FAA International Aviation Research and Development Symposium, April 1961

Good info on the interrogation, but how is the reply code entered and coded?  An additional 4096 codes would require just a single bit.  Was it an A/B or 0/1?  Where was the extra bit in the reply?

@Gerry: I believe the code would be entered like mode A/C (no additional digit), a selector would be switched to B on the transponder panel. There is no need for a 13rd pulse in the reply, ATC would know what to do with the code, knowing they interrogated in mode B. I read the L-1101 had such transponder with modes A and B, when flying in related areas.

That makes sense. So a pilot is told to "squawk Mode A, 4523" and another can get "squawk Mode B, 4523" and the ATC computers can track them without ambiguity as they are interrogated separately.

Mode B is also used as an antenna test pattern measurement within the FAA MSSR type systems. The MSSR interrogates a dual transponder test set with a mode B, which initiates the test pattern. This is used for troubleshooting and antenna health checks. Mode D is selectable as well, but currently not utilized.

"Mode B is also used as an antenna test pattern measurement" there is no need to use any information to establish the radiation pattern of an antenna, therefore it would be useful to provide a link to the documentation confirming the use of mode B by FAA, that would allow to understand the exact purpose. "Mode D is selectable" by who? how?


text:
Thinking of changing careers!
A little background: I am a senior software engineer with over 14 years of experience. I will turn 30 soon. I live in the UK but I'm neither a British citizen nor from EU member country. BSc in software engineering (abroad) and MSc in Advanced Computer Science (UK).
If I decide to pursue a career as a pilot, would I be eligible?
I'm not just asking this randomly, I genuinely want to know and seriously thinking of it. Aviation's been my dream for so long now I can't even remember the beginning of it. Not studying to become a pilot has been my biggest regret so far, and feels like this is one of those last moments I can do something about it. Not counting the satisfaction I can't get from my industry anymore - been doing this for some time now!
I hope this type of question is not against the rules of this community, but I don't know where else I can this. Also, if you were in my position, would you risk everything and do it?

My question is not exactly related to the age, but mostly about qualifications.

You should post this on PPrune Forums https://www.pprune.org/trending.php, probably the biggest insider gossip and information/discussion site.  You will get broader and more useful advice compared to here since it's an active community of professional flight crew and techs.

I am not sure it is the right moment to switch into a pilot career. Due to the Corona crisis there is a reduction in air traffic of 90% in Europe, passenger numbers are down by 97%. Remote working and video conferencing is being adopted at large scale and is here to stay. I don't think the aviation sector will fully recover from this crisis, since many business trips in the future will be replaced by online meetings. At the same time, unmanned / unpiloted aviation is developing rapidly. Why not consider a career as a software engineer in aviation?

If you get into training now, by the time you graduate there will be a shortage of pilots.

NUMBER ONE factor:  stay single and childless until you are working and making decent money as a pilot.

Your previous education does not matter that much, it's way more about your suitability for the extremely demanding job of professional pilot.
You can enter a range of differing training paths that lead, should you successfully complete the hard training, to an atp licence.
The ones that offer some kind of promise of getting hired, have rigorous selection processes.
Any training towards professional pilot license is ridiculously expensive, you'll have to consider that. It'll be years before it pays back financially.
Only you can be the judge of whether or not to pursue this, but a word of advice, if I may: it's a job, just like any else. It's really about how well you fit the job and vice versa. If you must become a pilot, you will.


text:
Are gyroscopic effects of tail rotor noticeable with hover spot turns?
I have a question of the book  "The foundations of helicopter flight" by Simon Newman.
The question is from page 186:

1) How can we feel gyroscopic effect of tail rotor with hover spot turns?

I suggest you split this question into four questions.

@SteveV. OK I edited it. Thanks for your comment

I'm sure this will kick off a war of words.  It is a myth that a helicopter rotor is a gyro.  It is not.  The blades fly to their position, they do not precess.  The phase angle changes with wind direction and for each blade.  The precession angle for a gyro is always, and must be, 90 degrees.  When the angle of attack of the blades is altered, the rotor thrust alters accordingly and acts through the plain of rotation.  If a rotor was a gyro, it would probably tear itself off when tilted in response to pitch changes.

Just thought of a good demonstration of why a rotor is not a gyro.  Watch a pilot doing a rotor disc flying check.  The rotor can tilt very quickly.  Now imagine a gyro of that mass and angular momentum and the forces involved in trying to move it from it's plane of rotation.  Wow!  I can't find a video anywhere but you can get quite "frisky" with the cyclic so long as the collective is firmly planted.

@Simon can you give a reference to where it is documented that it is not like a gyro as even experimental helicopter test-pilots confirm that it acts like a gyro. Even though the force doesn't act exactly 90° later, but perhaps 75-85°

@ChrisV How about Prouty, one of the leading experts on helicopter dynamics.  I've also read NASA pieces on this but my Google foo is temporarily weak.

Related: Helicopter swashplate movement

The tilt and the gyroscopic effect cause the helicopter to hover with a slight bank in a no-wind condition, a compromise that is acceptable. It should also be noted that a crosswind can exaggerate the bank, eliminate it, or even cause the helicopter to bank in the opposite direction.
In helicopters, the controls are rigged is such a way that when forward cyclic is applied, the helicopter moves forward, likewise for aft, etc.  To accomplish this, the pitch horn is offset 90º to the rotor blade.  The controls still tilt the swash plate in the same direction as the control input is made, but due to the pitch horn placement, the input to the blade occurs 90º earlier in the plane of rotation. The tail rotor as a gyroscope is rather insignificant
http://books.google.co.in/books?id=lTkiuG4RaFIC&pg=PA410&lpg=PA410&dq=gyroscopic+effect+%2B+tail+rotor&source=bl&ots=0t9R_FBArG&sig=pWRaKnWgjGAqkmeePDrmmclx7UA&hl=en&sa=X&ei=rSHhU7jSBMG0uAS9ioDIDQ&ved=0CD8Q6AEwCQ#v=onepage&q=gyroscopic%20effect%20%2B%20tail%20rotor&f=false
Also,  speed decreases all the way from the start of the approach to the hover point, and the power, controlled by the collective-mounted throttle, decreases during the first half of the approach, and then increases during the latter half - as the helicopter nears the hover spot and loses translational lift.
In a stable, no-wind hover everything is in equilibrium. The engine is providing just enough power to keep the main rotor turning at just the right rpm to hold the height over the ground and to keep the tail rotor spinning at just the right angle to counteract the torque of the main rotor. When you push one or the other of the tail rotor pedals forward, you upset this balance and must do something to compensate in order to hold the same hover height.
In a hover, the torque of the main rotor tries to turn the fuselage to the right and therefore pressure on the left pedal is required to keep the nose straight. Forward left pedal means the tail rotor blades are biting the air at a greater angle of attack and therefore producing more lift. If you add even more forward left pedal to initiate a left hovering turn, you increase the tail rotor blade pitch angle even more. This requires more power from the main gearbox and, if engine power remains constant, the only way the gearbox can satisfy this increased demand for power from the tail rotor is to allow main rotor rpm to decrease.
The tail rotor provides horizontal thrust but only to the right (nose left).  The tail rotor counters the torque on the fuselage caused by the engine driving the main rotor

Contrary to some misguided views, all helicopter rotors, both main and tail, are gyroscopic and precess at exactly 90 degrees to the sum of ALL applied moments. The control mechanisms of all helicopters are offset in phase to allow for that.
Because they are gyroscopic, rotors simply do not roll easily. The aerodynamic forces needed to roll the rotor are typically four or five times the forces needed to roll the hull. A violent roll may reflect in the helicopter's torque meter reading because of the extra power needed.
In the case of a tail rotor, when a hovering yaw is carried out, the tail rotor axis is being turned about a vertical axis and so it will precess about the roll axis of the helicopter.
This is accommodated using mechanical coupling in the tail rotor hub, typically by turning the axis of the teetering bearing by something like 45 degrees. When the tail rotor disc rolls, the coupling causes cyclic pitch changes on the tail rotor blades that oppose the roll. As a result the tail rotor follows the hull in a yaw instead of precessing and munching bits off the fin.


text:
How does IFF work?
How does Identification Friend or Foe (IFF) work? Are radar images of friendly planes recorded, or do friendly planes have transponders on them?

IFF systems use a transponder/interrogator combination to allow aircraft to issue challenges, receive responses, and do the friend or foe test. BAE Systems has a transponder product page with a little info, and Wikipedia (yeah yeah) has a reasonably thorough description and history of IFF.
According to NATO STANAG 4579 (abstract):

This NATO BRID is a millimetric wave question and answer system working in the Ka band. It comprises two main components, an interrogator, which allows a platform to question another platform with an encrypted message, and a transponder, which decodes the message and replies with an encrypted answer [emphasis mine]. Transponders will be fitted to all designated platforms, while interrogators will be fitted to only those designated platforms with an offensive or reconnaissance capability.

I assume these systems would be enabled in warzones. Wouldn't this give away the aircraft's position?

That's one of the reasons it's a cryptographic handshake. If the query doesn't decode properly, the transponder doesn't answer at all. Of course this means that the interrogator lights up for a moment, but there are ways to manage that... and realistically, if you're close enough that IFF matters, you're already on radar.

@keshlam However, if it does decode properly (i.e. is a friendly request), the transponder would light up as well, potentially giving away both their positions, wouldn't it? In this case they only need to be "close enough" to each other, not the enemy.

Call it the indeterminacy principle. A single ping giving current position doesn't say where it will be three seconds later, and that's presuming the ping can be recognized (I would assume it's spread-spectrum) and triangulated upon at all.

It looks like STONAG link is broken.


text:
Is there information available about an aircraft incident from New York to LHR in 2004?
I am attempting to find some form of incident log regarding a flight I took i 2004 (I believe May/June).  I believe it was a British Airways flight but it may have been operated by another airline.
I was on a flight from New York (I think JFK) to LHR when we had to divert to a runway in Bangor, Maine due to a cockpit fire, I seem to recall being caused by an Air Conditioning Unit.
I can't seem to find any details online about this incident.  Does anyone know of any resources I could use to locate this?  It was in the Bangor Daily News at the time but I've searched their site and can't find any info.
It seems to me that this wasn't "major" enough of an incident to be on any main logs, etc.

Do you know what airline at least? The NTSB database doesn't show any accidents/incidents at BGR (other than a 208 Hard Landing in 2003) from 2000 to 2006.

Concorde? http://www.airliners.net/forum/viewtopic.php?t=212653 (I'm guessing you would remember if a) it was going the other way and b) it had a very pointy nose cone)

Ha - No pointy nose and I was definitely flying home to the UK.  I'll try to dig out some more info and update the question.  I took photos of the fire engines so if I can find them I can find the date.

It seems (froma bit of googling) that Bangor is a very popular place for diversions between JFK and LHR. Any more detail you can provide may help narrow it down a bit

I have narrowed the date down to May-June 2004.

@Ste - just realised who you are from 12s/WH days from your avatar!!

@Jamiec And now I know who you are!  Small world!  The flight in question was coming back from the 12s meet in NYC!


text:
What is a "Lifeline Extension Belt"?
I was recently flying on an United Airlines A320 and noticed a narrow section between the overhead storage bins with a placard labeled "Lifeline Extension Belt" on it. It was over the wing exit row. What exactly is a lifeline extension belt?
Unfortunately I didn't get a picture, but what I'm referencing can be seen in these photos (not copied here since they are copyrighted):

https://www.flickr.com/photos/oliverporter/7653848312 (on the top-left)
https://www.flickr.com/photos/49111993@N00/13890509784 (narrow section is visible rising from the forward exit to the overhead compartment, no placard)

I think what the placard actually says is.

Lifeline
Extension Belt

(ie, its 2 things, not one!)
An A320 equipment specification indicates the narrow OHB you reference as number 16. Which is titled "Lifeline". This line is used to aid evacuation from the overwing exits and more details can be found in this answer
Extension belts are used for infants on their carers laps, an extra belt loops round the child and attaches to the carers lap belt.

(source: cabincrewsafety.com)

Actually extension belts are used for obese passengers and simply have a male and female fitting on it. The child restraint belt with an extra loop is banned in the US due to the risk that the adult crushes the child in an accident.


text:
How to start deriving longitudinal equations of motion for an aircraft?
I hope this is a relevant place for me to ask a math question regarding aircraft design.
I am trying to understand how one would implement a controller to control the pitch angle of an airplane for a small exercise. I understand the control part and its implementation. What I do not grasp is how one acquires the longitudinal equations of motions (which are then used for the control part) which serves as the starting point. What is the starting point or what are the principles used to derive these equations? If I know how to derive these equations for a very simple case, then I know I have to linearize the equations and then apply control theory to it.
For example, how are the left and right hand sides of eq. 4.70 from pp. 164 of the following book book is derived?
I will appreciate a simple explanation of the above case.
Edit:

I am attaching two screen shots of two sets of equations from two sources. Links to the books are included below. Both sources state these are longitudinal equations of motion although their general form differ from each other.
I think I got to understand one point: these equations were derived considering translation motion on the x and z planes and rotation about the y axis (so stated in the first book) Thereafter, I don't understand the procedure.

1st set of equations from book 1:

second set of equations from source 2:

book1: pg. 164 of Morris, Introduction to Aircraft Flight Mechanics: Performance, Static Stability
source 2: pg 3 of this online note

Not sure, but this document on NASA's website may help.  At least they explain their units.

Please include the equations in your question. I can probably help you but since I don't have the book there is little to start from.

@user1420 the two things which come to mind are the longitudinal stability formula and the first derivative of longitudinal stability formula, is it one of these?

It appears that I'm not able to view the equation from the book link that you've given. Could you provide an image or type it out?

I believe a PID controller with pitch or vertical speed as input and elevator displacement/trim as output with a bit of experimental tuning is good enough without needing any model. Early autopilots were even easier controllers; IIRC there was some rudimentary autopilot in WWII bombers, which had to be simple analog device.

@ Jan Hudec, I was going to do the opposite of what you mention: use elevator displacement as input and pitch as output. Thanks for your suggestion about the PID approach though. Since this is more of a learning exercise for me, I will also try this out.

@Manfred, As far I understand, its the longitudinal equation of motion. Please see the images I added.

Please see the images of equations I added later on.

may I suggest that the tag is not the most appropriate one?

@user1420 i'm working on an answer, i hope it will be of sufficient level. in the meantime, if you drop your email in chat (http://chat.stackexchange.com/rooms/12036/the-hangar) i can send you a few lecture slides etc.

@Manfred, please do email me any resources you have: achand007 (at) gmail (dot) com

I don't understand why this has a close vote on it, it is quite clearly on topic.

Note: Answer in progress!
Part 1
(Unfortunately I'm only familiar with #1 and #3 at the moment, not #2)
(Footnote: This might be a bit simpler than your case, but hopefully it you'll be able to fill in the remaining gaps)

From this, you can some the forces up according the direction of the velocity or the lift  vector. Doing this horizontally, you get equation 1, and likewise for vertical direction your equation 3.
To make this simpler to handle, we use small angle approximation consider $\cos(0)=1$ and $\sin(0)=0$. This simplifies down to:
$$V:0=T-D$$
$$L:0=mg-L$$
(i.e. thrust equal to drag, lift equal to weight)

Part 2:
This is basically the equations of a kinetic diagram of the Free Body Diagram above, where there can be a change either in airspeed of altitude. What your second equation says is that excess thrust (T-D, Thrust-Drag) can:

be used to increase altitude: $m\times{g}\times\sin(y)$

and/ or

be used to increase airspeed: $m\times{v}$

Depends on what you mean by "derive the equations". If you really mean that you want to work your way up to that formulation starting from the basics, well you start from good old Newton:
$$\overrightarrow{F}=m\overrightarrow{A}$$
and the equivalent for moments ($\overrightarrow{}$ indicates vectors).
At this point you need a reference system in which to decompose your vectorial equations (body-fixed, earth-fixed, stability axes: the choice influences which terms you'll be able to simplify later) and a description of your system: which forces are applied on the aircraft? how can I describe them as functions of the aircraft state?
At this point you substitute in your original equation and carry on the computation.
You can consult these lecture slides to see step-by-step how it can be done. Lecture 8 and 9 for the general 6DoF case and lecture 11 for your particular question.


text:
Do you ever get to practice ejecting out of a plane as a fighter pilot?
The military throws away a lot of equipment simply because it doesn't hold up to current standards. So I was wondering the following.
As a fighter pilot, would you ever get a practice run ejecting out of a plane, or is it just kind of one of those things you read about and hope you never have to do it? What is it like ejecting out of a plane at 500 knots? 1000?

I have no idea how accurate this article is, but if it is even roughly accurate, it suggests that serious injuries are not uncommon, which implies that it is not one of those things that are tested all the way through.

I'd wager a real ejection is definitely one of those things a pilot hopes to never have to do - While being blasted clear of an aircraft that's about to explode or crash is preferable to the alternative (dying) it's certainly not without its own risks.

I remember hearing that one of the pieces of advice given to those who may have to eject is to be sure and put your back in the position you would like it to be for the rest of your life.

If you are in a situation where you have to eject, you aren't expecting to come out without injuries.

Can't they just back flip the seat, so that the pilot does not have to face the acceleration in Mach 1 cases? Would that help?

@Firee No. The problem isn't that the pilot is suddenly exposed to high-speed winds in their face. The problem is that it is necessary to accelerate the seat vertically extremely hard to avoid the seat being struck by the rest of the plane.

Air forces use special training rigs rather than real ejection seats. Example video

Plenty of pilots have survived ejection (Wikipedia says "As of 20 June 2011 ... the number of lives saved by Martin-Baker products was 7,402 from 93 air forces")
The types of injury are illustrated by this example

The ejection seat has been responsible for saving the lives of thousands of pilots around the world since its introduction in the late 1940s. Typical survival rates quoted in the literature vary from 80–97%.
It is generally accepted that radiographic evidence of vertebral fracture can be found in 30%–70% of aircrew after ejection

The reason real ejection seats are not used is almost certainly the high risk of injury. Ejecting clear from a high speed jet requires a lot of acceleration.

Just a couple things to think about.  The parachutes used aren't anything like what your skydive with.  You come crashing back to earth around 30mph.  Broken legs and ankles are very common. Flail injuries are all but guaranteed during a high speed ejection.

Cons: broken ribs, broken ankles. Pros: not exploding in a jet fueled inferno.

Ejecting at 1000 knots means ejection into supersonic flow. Not even the Zvezda K-36, the indisputably best ejection seat around, is rated for that. For this kind of speed, whole enclosures have been designed which did not work all too well when they had to be used.

For training, there are simulators to prepare pilots for that hopefully rare occurrence (the picture above is from this link; there you'll find more pics and a list of their customers), but mostly, simulations are run with simulated pilots, too. The spinal loads are too high to have pilots exposed to them regularly without the benefits which go with a successful ejection.
For the things that might happen, see what William Rankin went through on July 26, 1959. It can last quite a while, too. In his case 40 minutes.
By the way, ejection seats started to save pilot's lives in 1942 (Germany) and 1945 (Sweden).

On the other hand, the SR-71 used an ejection seat instead of a capsule. The crew wore full pressure suits, though, which provided similar benefits (one test pilot had his aircraft break up on him [he didn't eject, and was unconscious when he left the plane], and survived)

I particularly like this site

What is it like ejecting out of a plane at 500 knots? 1000?

Look for this story on the linked site... A USAF F-15 crew ejected, at night, estimated well over mach 1. The back seater was killed in the ejection; the pilot survived; wreaked havoc on his musculoskeletal system and required the lower part of one leg to be amputated but eventually returned to flying status.

As a fighter pilot, would you ever get a practice run ejecting out of a plane

My experience in a ejection trainer, much like the "air force training rig" above, gave a very short initial jolt equivalent to the real thing. I had my head bent ever so slightly and I had a sore neck for weeks. There was never a repeat ride in our year long training.
Ejecting out of the real thing for training? We would have called that sort of thing "practice bleeding".

An SR-71 pilot survived an ejection during a mid-air breakup at 2000mph

There are ejection seat trainers, most concentrate of the required procedure before ejection and upon successful completion do a short 10 cm hitch upward, only a jolt to indicate end of procedure.

Some air forces do have trainers that give more of a ride. I got a demo ride in the sim in the picture, since I was involved in supplying some of the hardware. It was a severely toned down ride with much lower accelerations than pilots experience for real, but still too much for my blood pressure: one moment I was giving the all clear to the instructor, the next moment I was 3 metres in the air with an aching neck. I have no recollection of what happened in between.


text:
Can I fly with an airband receiver in the United States?
I'm flying with American Airlines on a cross-country flight soon and was thinking about bringing along my airband receiver/scanner since it'd be fun to listen to ATC.
It looks like I'd be unable to use it during the flight since any type of radio isn't allowed. I don't see any TSA restrictions on them. Am I safe in assuming that I can bring it along and listen to it in the airport terminal without any trouble with security?

It's probably easier to head over to LiveATC and bring up the relevant feed for your area.

I have brought a scanner along on many trips by plane. I've never had any trouble getting through security or using it in the terminal.

This is probably more a travel question than an aviation one, but I did some Googling of various aviation and travel sites and the consensus seems to be this:

You can carry the scanner with you, but if you're unlucky and encounter the wrong TSA agent on the wrong day then you may be asked a lot of questions at security (the TSA's "Can I Bring..." tool seems to consider scanners to be generic electronic items)
Using the scanner while sitting in the airport is fine, but if you attract attention to yourself then you may be questioned by the police or airport security
You may not use a two-way radio on board the aircraft, because they're specifically required to be off at all times (this is in every airline's safety instructions that I've seen); that shouldn't apply to a radio that only receives but the cabin crew might ask you to turn it off anyway if they're not sure about it
Even if you did operate the scanner in the air, the reception is terrible and you'll probably hear only the aircraft's crew, not ATC

Note that this information is for the USA only, many other countries are very strict about possession and use of scanners or two-way radios and in the UK (for example) just listening to ATC is actually illegal.

Scanners are definitely NOT 2 way radios, they're strictly receive only. I've never been questioned by anyone using one at an airport, but I can imagine in the post-911 climate some rentacops might get pushy. You can get ATC in the air, no problem. Of course you're going to have shorter reception range than the equipment in the aircraft so you might not hear them until some time after the aircraft started talking to them.

@jwenting Yes, good point, although 3 out of the 4 points I mentioned still apply to a pure receiver. But I'll edit my answer to make that clearer.

Most airlines I have flown have restrictions on radio receivers during all phases of flight.  eg. http://www.united.com/web/en-US/content/travel/baggage/devices.aspx  I see some now appear to permit AM/FM receivers specifically.

Get a scanner that includes the FM broadcast band (88-108 MHz). If anyone questions you just tune to a FM broadcast station.

@BowlOfRed I'm not sure how common United's restrictions are nowadays. Delta, for instance, does not have those restrictions: http://www.delta.com/content/www/en_US/support/faqs/during-your-trip/personal-electronic-devices.html . Delta specifically states that AM radios are allowed and aviation comm radios just happen to be AM radios. :)

from an electronics standpoint, its an "AM" radio, but I'm sure that's not the definition they mean. They mean 540–1610 kHz MF.

@reirab I know from personal experience that KLM and BA explicitly state that "all equipment with an antenna needs to be switched off completely or be put in airplane mode", iow the circuits powering the antenna must be disabled.

@jwenting "Airplane mode" doesn't necessarily mean that all radios must be disabled. It does mean that cellular radios will be disabled, but other radios - both transmitters and receivers - can remain active. Both BA and KLM offer onboard WiFi for passengers to use, so they apparently don't require all radios to be disabled in flight.

Unfortunately, this answer is wrong and gives bad advice. You cannot use a AM/FM radio on board an aircraft in flight and therefore a scanner. Read the safety card. The issue is a phenomena called "IF Leakage", where the internal oscillators of the radio leak out the antenna, and can cause interference.

In the US, it's perfectly legal to bring an airband receiver or scanner through security and on to an airplane. There are restrictions on using radios (even if receive only) onboard an aircraft. Furthermore, you are required by the FAA to comply all pilot and flight attendant instructions (and they usually say turnoff all radios).
There is however a possible loophole by using a crystal radio. There's no rule against taking one onboard an airplane, and since there's no battery, it's impossible to turn off. Do a search for "airband crystal radio" for possible receivers and designs.

Interesting.... I found instructions for making one here (but it requires a 9V battery), but Wikipedia confirms batteries aren't required. Not sure if reception would be good enough inside the aircraft though, and it seems like such a hand-made device could attract some unwanted attention from the TSA.

@Jeff: You could put it in a more normal looking enclosure than the black cylindrical "pipe-bomb" enclosure shown in those instructions. Good-looking two-part plastic rectangular cases are inexpensive and readily available.

The only special restriction regarding the use of a radio on an aircraft in the U.S. is that the FCC bans the use of cellular radios in flight. All other types of radios are allowed, provided that they're otherwise operated according to FCC regulations and aren't prohibited by the airline or crew. Also, just because a crystal radio doesn't need a battery doesn't mean you can't turn it off. Opening the right part of the circuit (whether by switch or disconnection of the wires) will indeed turn it off.

I've been an avid shortwave listener since the 1950's, a ham radio operator since 1965, and I later retired as a USAF pilot.  Ever since the 60's when most everything  went solid state and handheld radios (both scanners and two way) started to have VHF aviation frequency, and later, UHF frequency (military aviation freqs) capability, I have always traveled with a handheld radio while flying on either commercial airlines or military hops worldwide.  In several decades of doing this I have never, ever had any issues with either security (TSA, etc.), flight crews, or customs personnel, even with radios capable of transmitting on aviation frequencies.  But, just in case, I always carry copies of my credentials with me (both my ham and FAA), which are valid worldwide. In addition, while on board I don't "advertise" my activities, placing my handheld in either my pocket or the seat magazine storage pocket and use a headset (with no microphone!). Quite often, when either changing frequencies or using the LOC/ILS to monitor an approach someone seated near me will be curious and ask me a question and I will answer their question in great detail and let them listen in or watch if they want to.  This serves to alleviate any concerns that they may have while also helping them better understand Ham Radio, shortwave listening, and other technical aspects of flying and the evolution of radios and the frequency spectrum at the same time. But hey, the secret is to just act normal, be cordial, and always do whatever a flight crew member may ask of you, if requested.  For between $150-350 some of the more modern handheld radios made by Icom, Vertex, or Sporties Flight Shop will provide you with hours of entertainment on board and help generate new friends at the same time.  Enjoy!

It is illegal to use a radio receiver in flight, check the safety card. As a Ham, you should know why: receivers all tend to use the same 10.7 MHz and 455 kHz IF. Finite shielding and limited isolation of mixers means a portion of each LO and IF will leak out the antenna and be received by the aircraft radios. This will then beat with the desired LO and signals and cause degradation of performance.

A passive crystal set with good front-end bandpass filter (Chebyshev 10-pole with 3 dB points at 118 and 136 MHz), a sensitive Schottky detector and hi-z phones or 32 ohm buds with audio matching xfmr, the headphone cord serving also as antenna (trim cord to 1/4 wave resonant length mid-band, coupling/decoupling via caps & RF chokes) and you're in business.
Use a metal case for a counterpoise (w/body capacitance coupling); insulate the headphone cord shield from case.
No batteries, no tuning, no on-off switch, no volume control, no interference potential. It's never "on", power supply-wise, so nothing to turn off.
This is the best way to hear all in-band activity without tuning around, missing brief transmissions in the process--think of it as a scanner that gets everything at once. Segregation is by volume; e.g. closer transmissions are louder, so your pilot will be vs. ATC.
For ultimate 'stealth', build it into a tiny, innocuous-looking case like from a gutted mp3 player (if questioned why it doesn't work, say "needs charging"), keep it in your pocket, and use only one earphone--cut the other one off. Keep that one on the side of your body facing away from the aisle for less visibility (window seats rule!) That way you can also  still hear flight crew instructions, etc.

Hi, Alfredo. We're not a discussion board so I edited out your response to another comment and your off-topic psychology claims. I'm still not entirely convinced that this really answers the question, though. I guess you're claiming that a device of the kind that you describe would be legal to use during the flight. Note that, at the moment, you might not be allowed on the plane with an electronic device that's "not working", though that's more likely for larger devices such as laptops and tablets.

The problem is... if you are asked/told to turn it off, whether it's an airband radio or a passive device then you HAVE to turn it off. Cabin crew don't want you listening to the pilot's radio traffic particularly if any kind of emergency arises or flight detour. Think about it!

Can you provide a source to support the assertion that a significant number of cabin crews would disapprove of you listening to radio traffic? Or are you assuming they don't because you wouldn't want that?

The problem is... if you are asked/told to turn it off, whether it's an airband radio or a passive device then you HAVE to turn it off. So, exactly how do you turn off a radio that is impossible to turn off?

bonkers. Sure, you HAVE to turn it off if told so by the crew. But there's no "most don't want you listening in".